This invention relates to a method for controlling operation of a multistage hydraulic machine, and more particularly, a method for controlling and adjusting a load or water level in a steady operating condition of a multistage hydraulic machine in which all pressure stages are interconnected through return passages and the highest and lowest pressure stages are provided with movable guide vanes.
In a known hydraulic machine, operation or running condition thereof was controlled by adjusting water flow amount passing through a runner, this adjustment being done by guide vanes provided on the periphery of the runner or by an inlet valve located at an inlet portion of the hydraulic machine. Regarding the known multistage hydraulic machine in which respective pressure stages are provided with runners connected through return passages respectively, there have been proposed several methods for controlling water flow conditions at the respective stages by guide vanes located on the peripheries of the runners of the stages thereby to control the operation of the hydraulic machines. However, with the multistage hydraulic machines of the type described above, it is extremely difficult because of constructional limitation to provide guide vanes on the outer peripheries of the runners of the corresponding stages and to control the opening or closing of the guide vanes by a guide vane control unit which is connected to the guide vanes, which provides a problem for practical use of the multistage hydraulic machine.
In the other controlling method in which only stationary guide vanes having a constant degree of opening is usually located on the outer peripheral surface of a runner of each pressure stage and the adjustment of the water flow amount and the machine operation control are performed by open-close control of the inlet valve provided at an inlet portion of the hydraulic machine. However, in this method, the water flow condition at the periphery of the runner of each stage cannot be properly adjusted in response to the flow amount of water passing therethrough, so that in low and high flow-rate ranges apart from design points, hydraulic performance lowers and the hydraulic machine is operated under lowered overall hydraulic performance.
In order to obviate defects or problems described above, there has also been proposed a method for controlling operation of a multistage hydraulic machine, in which movable guide vanes are provided only for the highest pressure stage and the flow amount of water can be adjusted by regulating the degree of opening of the movable guide vanes. According to this operation control method, the water head loss at the inlet valve which is observed in a case where no movable guide vanes are arranged at the highest pressure stage was not observed, so that this control method has more excellent operation efficiency than the first mentioned method. However, regardless of such advantages as described above, in the later mentioned method, since the lowest pressure stage is operated at an operation point largely apart from a designed operating point during operation with a small amount of water, water separation phenomenon and/or secondary local flow may often occur, and these adverse phenomena are often observed as water pressure on the outlet side of a runner lowers, so that there are such problems as that the water separation phenomenon and the secondary local flow increases at the lowest pressure stage where the water pressure on the outlet side of the runner of this stage is lower than those of the other stages, and the hydraulic machine of this type is operated under an unstable condition causing cavitation, strong vibrations and noises.
Accordingly, it has been considered to construct a multistage hydraulic machine provided with movable guide vanes for the highest and lowest pressure stages as an ideal multistage hydraulic machine having reasonable structure and relatively high hydraulic operation performance.
With the actual operation control for running the multistage hydraulic machine in which the highest and lowest pressure stages are provided with the movable guide vanes, in consideration of the load or water level adjustment control at the steady operation, the multistage hydraulic machine of this type has a complicated flow passage in comparison with a single-stage hydraulic machine and it is necessary to accurately operate or control movable guide vanes of the highest and lowest pressure stages. Moreover, in a case where the control of the movable guide vanes is not accurately performed, head ratio or distribution on the runners of the respective stages becomes non-uniform, which may cause the lowering of the hydraulic performance of the machine and generate cavitation, vibration and noises at the lowest pressure stage.